1. Field of the Invention
The invention relates to an electronic component mounted structure comprising an AC coupling capacitor, which functions to cut off signals with a predetermined frequency or less, mounted between two semiconductor elements.
2. Description of the Related Art
A conventional electronic component mounted structure comprising a plurality of electronic components such as semiconductor elements mounted on a circuit board is shown as an electronic component mounted structure 161 in FIG. 1. In this electronic component mounted structure 161, a semiconductor laser (LD) module 163 for transmitting a light signal, a photodiode (PD) module 164 for receiving a light signal, and an LD driver (LDD) 165 for driving the LD module 163 are mainly mounted on a circuit board 162.
The transmission distance L between the LD module 163 and the LDD 165 is previously designed to be a predetermined length from the viewpoint of preventing reflection of signals between the LD module 163 and the LDD 165. The transmission distance L is preferably as short as possible although it is restricted by other electronic components mounted in the electronic component mounted structure 161.
To this end, an impedance matching line (microstrip line) 166 having a predetermined characteristic impedance is provided between the LD module 163 and the LDD 165 provided on the surface of the circuit board 162. An AC coupling capacitor 167 for cutting off signals with a predetermined frequency or less is mounted on the impedance matching line 166 between the LD module 163 and the LDD 165.
As shown in FIG. 2, the circuit board 162 is a multilayered board having a multilayer structure comprising an impedance matching line 166, an insulator layer 168, a ground (GND) layer 169 and the like stacked on top of one another.
The electronic component mounted structure of this type is disclosed in Japanese Patent Laid-Open No. 174443/2000.
In the electronic component mounted structure 161, however, since the circuit board 162 is a multilayered board, the distance t between individual layers (thickness of the insulator layer 168) is very short (for example, about 0.2 mm). Therefore, a parasitic capacitance C18 as shown in FIG. 3 is disadvantageously formed between the AC coupling capacitor 167 and the GND layer 169 located just beneath the AC coupling capacitor 167.
Upon the formation of parasitic capacitance C18, a high frequency component of a signal is run away through the parasitic capacitance C18 into the GND layer 169. As a result, Tr (rising)/Tf (falling) in the eye aperture (degree of opening of eyes in signal waveform) is so slow that the eye is disadvantageously closed (dull).
The eye aperture is one of measures for the evaluation of waveform quality in digital signal waveforms. When the eye aperture is large, signal discrimination is reliable and, thus, the frequency of transmission error is reduced. On the other hand, when the eye aperture is small, the signal discrimination is unreliable and, thus, the frequency of transmission error is increased. That is, closing of the eye deteriorates transmission characteristics.
Further, the electronic component mounted structure 161 has a problem that, for example, an error in mounting the LD module 163 onto the circuit board 162 makes it difficult to provide a transmission distance L as designed.